This invention relates generally to fluid flow control valves and more particularly to fluid control valves having a variable fluid orifice.
The speed of most hydraulic motors and actuators is regulated by the rate of fluid flow through an associated hydraulic circuit. Some type of speed control is required in nearly all hydraulic motors and actuators, and fluid flow control valves are therefore widely used in hydraulic circuits.
Many such valves maintain a constant fluid flow by maintaining a constant pressure drop across a fixed orifice. This pressure drop is directly proportional to the fluid flow rate through the orifice. Maintaining a constant fluid flow rate across a fixed orifice in most hydraulic systems is difficult because of the variables associated with these systems. For example, the changing load characteristics of an associated motor or actuator may cause downstream pressure to fluctuate. Similarly, upstream pressure may fluctuate as a result of the operation of upstream system components.
Typically, fluid flow control valves monitor upstream and downstream pressure and compensate for pressure variations, thereby maintaining a constant pressure drop through the valve. Many fluid flow valves presently used in hydraulic systems compensate for pressure variations by throttling fluid flow. A typical throttling type pressure compensated flow control valve includes a fixed metering orifice and a coil spring biased piston or spool. Fluid pressure upstream and downstream of the metering orifice is vented to opposing surfaces of the balanced spool. Fluid flow is throttled as the piston moves in response to pressure variations at the opposing surfaces.
It is desirable in spool valves to have a constant spring biasing force acting on the spool at all operating positions. However, coil spring biased spool valves have inherent efficiency problems because of spring inaccuracies. Spring forces do not remain truly constant as the spring is stretched or compressed. Attempts have been made to compensate for these spring inaccuracies. For example, additional surface areas may be provided on the spool for exposure to pressurized fluid when the spool has been displaced a predetermined amount. However, such compensation methods themselves are often inaccurate and can unnecessarily complicate the valve manufacturing process.